1. Field of the Invention
The present invention is directed to composite filters and methods for preparing same. More specifically, it is directed to filter laminates of multiple discreet layers of material bonded together, with at least one of the layers being an asymmetric membrane.
2. Description of the Related Art
Composite filters are filters having multiple layers, and are useful in a variety of separations applications. In many cases, the various layers of a composite filter each impart different desirable properties to the filter. For example, in some applications, an extremely thin membrane may have advantageous flow rates in separations of very small particles, gasses, and the like. Yet such a thin membrane may be fragile and difficult to handle or to package into cartridges. In such cases, the fragile, thin layer membrane may be combined with a backing or with a stronger, more porous membrane, to form a composite having improved strength and handling characteristics without sacrificing the separations properties of the thin layer membrane. Other desirable properties imparted by laminating one membrane to another media may include increased burst strength, increased thickness, providing prefiltration capability, and providing an adhesive layer to ease assembly of a device.
A problem with some composite filters is that the layers may tend to separate in use, adversely affecting the strength and performance of the composite. This problem has been addressed in different ways. In some cases, the layers of desirable composites are laminated together to create bonds between the layers that assist in preventing layer separation (delamination). An example of such a membrane laminate is provided in U.S. Pat. No. 5,154,827. That reference describes a polyfluorocarbon microporous membrane made up of three or more sheets of aggregated microporous fluorocarbon polymer. A fine porosity sheet is laminated between sheets of larger porosity microporous fluorocarbon polymer. A mixing liquid or lubricant is layered between the sheets to facilitate binding and lamination of the sheets to each other, and the stack is laminated into an integral composite membrane under application of heat and pressure. Exploiting the strength provided by the outer layers, the laminate thus formed can be pleated and packaged into filter cartridges.
A different approach to making composite membranes is to cast or form one membrane layer in situ on top of another layer. The base layer may be a fibrous backing material or it may be a membrane. U.S. Pat. No. 5,240,615 discloses a smooth microporous polyvinylidene difluoride (PVDF) membrane laminated to a porous support. A PVDF-containing dope is applied to the porous support and then gelled to form the supported PVDF membrane. A primary advantage for this process, as disclosed in the ""615 patent, is that the support prevents shrinkage of the PVDF material during gelling and drying. U.S. Pat. No. 5,433,859 discloses a supported microporous filtration membrane having a support layer with two different zones of microporous membrane being formed thereon. The membrane is made by applying a first casting solution onto the support layer, and then applying a second casting solution on top of the first. Both casting solutions are quenched simultaneously to form the supported membrane. This process forms a continuous, supported microporous membrane with two zones. The fibers of the support layer may penetrate into the adjacent membrane zone, but do not reach the second (top) membrane zone.
One of the primary benefits of composite membranes has been to provide a strong filter material having a relatively low resistance to flow. The greatest resistance to flow occurs in the region with the smallest pores. A composite of a very thin filtration membrane supported by a thicker, more open membrane thus minimizes flow resistance while maximizing strength. In addition, the support material may act as a prefilter, if the support material of the composite is upstream of the minimum pore material. The prefilter effect is especially beneficial in applications requiring a high dirt holding capacity, such as filtration of high particulate solutions, pyrogen removal, sterilization applications, and the like.
The advent of highly asymmetric polymer filters provided an improvement over composites for many applications requiring high flow rate and high dirt holding capacity. U.S. Pat. No. 4,629,563 discloses highly asymmetric microporous membranes with pores on one surface of the membrane having an average diameter 10 to 20,000 times smaller than the pores on the other surface of the membrane. The support layer between the membrane surfaces has flow channels whose diameters generally increase gradually in size along the distance from the minimum pore surface to the maximum pore surface. In the highly asymmetric membranes of the ""563 patent and subsequent patents, the smallest pores reside in a relatively thin layer near one surface, and this thin layer of minimum pores thus offers little flow resistance, while the membrane as a whole exhibits the strength and high dirt holding capacity that had previously only been available with composites.
Thus, prior to the present invention, highly asymmetric membranes were seen as a very attractive alternative to composite membranes. The invention disclosed herein represents an advance in composite membrane technology, and a new application for highly asymmetric membranes.
In a first embodiment of the present invention, a filter laminate is provided including a plurality of discreet layers of material, each layer being adjacent at least one other layer, wherein at least one layer is an asymmetric membrane and at least one layer is a hot melt adhesive, the laminate including a bond between each of the adjacent layers, wherein the bond is formed after the formation of the material of the layers. The asymmetric membrane may have a first and a second surface, each of the surfaces including pores, wherein the pores of the second surface have an average diameter at least about 5 times greater than an average diameter of the pores of the first surface, more preferably 10 times greater. The asymmetric membrane may further include a support structure between the first surface and the second surface, wherein the support structure includes a reticular network of flow channels connecting the pores of the first surface with the pores of the second surface. The flow channels may generally increase gradually in diameter between the first surface and the second surface.
In another aspect, the asymmetric membrane includes an isotropic region and an asymmetric region, such that the support region includes a thickness between the first and second surfaces, wherein the thickness includes the isotropic region between one of the surfaces and a point within the support region, and an asymmetric region between the point and another of the surfaces, wherein the isotropic region includes flow channels that are substantially constant in diameter from the surface adjacent the isotropic region to the point between the isotropic region and the asymmetric region, and wherein the asymmetric region includes flow channels that gradually increase or decrease in diameter from the point to the surface adjacent the asymmetric region.
In a further aspect, the filter laminate includes an asymmetric membrane wherein the average diameter of the pores of the first surface thereof is between about 0.01 xcexcm and about 10.0 xcexcm, more preferably less than about 0.01 xcexcm. The filter laminate may further include a first asymmetric membrane as a layer, and a second membrane as a distinct layer. The second membrane may be an asymmetric membrane including a first and a second surface, each of the surfaces including pores, wherein the pores of the second surface have an average diameter at least about 5 times greater than an average diameter of the pores of the first surface. The first asymmetric membrane layer may be bonded to the second asymmetric membrane layer. The first or second side of the first asymmetric membrane may be bonded to the first or second side of the second asymmetric membrane.
In a further aspect, at least one of the membranes includes a polymer additive on a surface thereof, the polymer additive contributing to the bond between the membranes. The polymer additive may include polyvinylpyrrolidone or polyethylenevinylacetate.
In a further aspect, the membrane is contacted with an adjacent layer and a bond is formed therebetween, wherein the membrane is wet when contacted with the adjacent layer, prior to formation of the bond. The bond between the adjacent layers may be formed in the presence of a temperature higher than a melting point of a component participating in the bond, and lower than a melting point of the asymmetric membrane.
In a further aspect, the asymmetric membrane of the filter laminate includes a polymer such as polyvinylidene fluoride, polyarylsulfone, polyethersulfone, polyarylsulfone, polyamides or cellulosic derivatives.
In a further aspect, the material of at least one of the layers of the laminate is polyester, polypropylene, polyolefin, polyethylene, nylon, paper, cellulose, glass fiber, acrylic, nonwoven fibrous material, woven fibrous material, web, sheet, calendared, wet laid, dry laid, and extruded material. If the material is a sheet, the sheet may be liquid-impermeable.
In a further aspect, the filter laminate has a total thickness of less than about 1000 xcexcm, more preferably less than about 500 xcexcm, and most preferably between about 75 xcexcm and about 350 xcexcm.
In a further aspect, the filter laminate includes a hot melt adhesive, including thermoplastic, polyester, nylon, ethylenevinylacetate, polypropylene, polyethylene, web, nonwoven material, woven material, powder, and solution hot melt adhesives.
In a further aspect, the filter laminate includes an asymmetric membrane that is cationically charged, anionically charged, hydrophobic, hydrophilic, or oleophobic.
In a second embodiment of the present invention, a filter laminate is provided including a plurality of discreet layers of material, each layer being adjacent at least one other layer, wherein at least one layer is an asymmetric membrane including polyvinylidene fluorides, polyamides, and cellulosic derivatives, the laminate including a bond between each of the adjacent layers, wherein the bond is formed after the formation of the material of the layer
In a third embodiment of the present invention, a filter laminate is provided including a plurality of discreet layers of material, each layer being adjacent at least one other layer, including at least a first asymmetric membrane as a layer, and a second membrane as a distinct layer, the laminate including a bond between each of the adjacent layers, wherein the bond is formed after the formation of the material of the layers.
In a fourth embodiment of the present invention, a method of making a filter laminate is provided, including the steps of providing a first plurality of discreet layers of material; contacting the layers to form a first stack, wherein each layer is adjacent at least one other layer in the stack; forming a bond between adjacent layers in the first stack, wherein the bond is formed after the formation of the material of the layers, thereby forming a first laminated stack layer; contacting the first laminated stack with a second layer of material; and forming a bond between the first laminated stack layer and the second layer, wherein the bond is formed after the formation of the material of the layers, thereby forming a filter laminate.
In another aspect, the second layer includes a plurality of discreet layers, wherein at least one of the discreet layers includes an asymmetric membrane.
In a further aspect, the method further includes the step of forming a bond between adjacent layers in the second layer, wherein the bond is formed after the formation of the material of the layers, wherein the bond is formed before the step of forming a bond between the first laminated stack layer and the second layer.
In a further aspect, the method further includes the step of forming a bond between adjacent layers in the second layer, wherein the bond is formed after the formation of the material of the layers, and wherein the bond is formed substantially simultaneously with the step of forming a bond between the first laminated stack layer and the second layer.
In a further aspect, a bond is formed by heating a stack or a layer to a temperature of about 200xc2x0 F. or less, a temperature of about 200xc2x0 F. to about 395xc2x0 F., or a temperature of from about 396xc2x0 F. or higher.
In a fifth embodiment of the present invention, a method of making a filter laminate is provided including the steps of providing a plurality of discreet layers of material, wherein at least one layer is an asymmetric membrane and at least one layer is a hot melt adhesive; contacting each layer with at least one other layer to form a stack including at least two layers; and forming a bond between adjacent layers, wherein the bond is formed after the formation of the material of the layers, thereby forming a filter laminate.
In a sixth embodiment of the present invention, a method of making a filter laminate is provided including the steps of providing a plurality of discreet layers of material, wherein at least one layer is an asymmetric membrane including polyvinylidene fluorides, polyamides, or cellulosic derivatives; contacting each layer with at least one other layer to form a stack including at least two layers; and forming a bond between adjacent layers, wherein the bond is formed after the formation of the material of the layers, thereby forming a filter laminate.
In a seventh embodiment of the present invention, a method of making a filter laminate is provided including the steps of providing a plurality of discreet layers of material, wherein at least one layer is an asymmetric membrane and wherein at least one layer including polypropylene, polyolefin, polyethylene, nylon, paper, cellulose, glass fiber, or acrylic; contacting each layer with at least one other layer to form a stack including at least two layers; and forming a bond between adjacent layers, wherein the bond is formed after the formation of the material of the layers, thereby forming a filter laminate.
In an eighth embodiment of the present invention, a method of making a filter laminate is provided including the steps of providing a plurality of discreet layers of material, wherein at least one layer is an asymmetric membrane and wherein at least one additional layer is a membrane; contacting each layer with at least one other layer to form a stack including at least two layers; and forming a bond between adjacent layers, wherein the bond is formed after the formation of the material of the layers, thereby forming a filter laminate.
In a ninth aspect of the present invention, a method for filtering ink is provided, the method including providing a filter laminate, the filter laminate including a plurality of discreet layers of material, each layer being adjacent at least one other layer, wherein at least one layer is an asymmetric membrane, the laminate including a bond between each of the adjacent layers, wherein the bond is formed after the formation of the material of the layers; and passing an ink through the filter laminate, whereby the ink is filtered.